This invention relates to fused cast ceramic refractories, and in particular to treatments to reduce erosion of such materials and their tendency to introduce "stone" defects into the product when they are employed as structural members in contact with molten materials. Fused cast ceramics are characterized by methods of manufacture that include melting a mixture of oxides to a molten, amorphous condition, casting the molten mixture into a mold, and as the material cools permitting crystallization to take place within a glassy matrix. The crystalline ceramic phase is highly refractory and the glassy phase helps accommodate thermal expansion. Thus, fused cast ceramics lend themselves to use in high temperatures applications, and in particular where contact with molten material such as molten glass is involved.
The glassy matrix that characterizes the type of ceramic materials with which the present invention deals imparts advantageous properties to the material but also is responsible for a mechanism for failure. Since the glassy phase is less viscous than the crystalline phsae or phases, exudation of the glassy phase from the surface of the refractory over a period of time at operating conditions can occur, thereby accelerating erosion of the surface of the refractory member. The crystalline phases can be selected to have very low rates of dissolution in the molten material in contact with the ceramic, but loss of surface portions of the glassy phase substantially increases the surface area of the crystalline phase that is exposed to the erosive action. Additionally, exudation of the glassy phase increases the frequency with which particles of the crystalline phase are freed from the surface and enter the melt. Because these particles are highly insoluble, they are slow to dissolve and often appear in the final product of the melting process. In glassmaking, these undissolved particles are known as "stones" and are considered highly detrimental to the quality of the glass. Discarding portions of the glass with stones can seriously reduce the productivity of a glassmaking operation. Therefore, it would be highly desirable to provide fused cast ceramic refractory materials that would have greater resistance to glassy phase exudation whereby erosion would be reduced to extend the life of the refractory structure and to reduce the production of stones. The improvements sought would be particularly beneficial in the glass industry, but advantages such as reduced erosion should have applicability to a wide range of applications.
In the prior art, sintering of ceramic materials has employed methods that have similarities to the present invention. But sintering and the problems and objectives of sintering are basically different from fused cast ceramics. Sintering involves molding a mixture of ceramic particles into a desired shape and then heating the molded article until the surfaces of the particles soften sufficiently to bond together. A molten state and phase separation are not involved in sintering. Because sintering involves bonding discrete particles together, a common problem is entrapment of air or other gases in the interstices among the particles. Therefore, sintering is sometimes accompanied by measures to remove gases. Additionally, molding of the article prior to sintering often involves mixing the particles with water or other carrier liquid or organic binders. Removal of these liquids or binders or their decomposition products is another motivation for sintering to involve degassing measures. Providing a special atmosphere is also involved with some sintering techniques for the sake of avoiding oxidation or other unwanted chemical reactions. None of these factors is involved in the production of fused cast refractories since the rendering of the raw materials to the molten state would seem to preclude any concern with interstitial gas entrapment. Also, fused cast materials do not have the relatively high porosity and permeability that characterize sintered materials, so that there has been little concern with the presence of gases in fused cast refractory products.
U.S. Pat. No. 670,299 (Schwarz) involves the imposition of vacuum during a cementitious reaction to prevent air from interfering with the reaction.
U.S. Pat. No. 1,615,022 (McCullogh) discloses baking a ceramic material onto a metal member in vacuum to avoid oxidation and to reduce porosity of the ceramic.
U.S. Pat. No. 1,803,355 (Reichmann) relates to a process for sintering aluminum oxide in vacuum to avoid forming aluminum carbide.
U.S. Pat. No. 2,128,289 (Dubilier) discloses sintering ceramic mixtures in vacuum to prevent deoxidation of titanium dioxide.
U.S. Pat. No. 1,512,801 (Richardson et al.) discloses treating a slip for casting refractory ware in vacuum to remove air bubbles prior to casting and firing.
U.S. Pat. No. 3,859,405 (Horton) involves a process for sintering ceramic articles in which a binder is removed under vacuum during moderate heating prior to sintering.
U.S. Pat. No. 3,343,915 (Rossi et al.) discloses removal of solvent from metal oxide powder by vacuum prior to sintering.
U.S. Reissue Pat. No. 20,460 (Jeffery) involves slip-casting of ceramics in which voids are eliminated by pressing the casting prior to firing.
U.S. Pat. No. 3,954,930 (Vasilos et al.) discloses hot pressing of ceramics under vacuum to remove trapped gases.
U.S. Pat. No. 4,242,294 (Huther et al.) teaches sintering a glass or ceramic coating under vacuum.
U.S. Pat. No. 4,294,788 (Laskow et al.) discloses molding silicon carbide under vacuum to avoid oxidation.
U.S. Pat. No. 4,396,572 (Batigne et al.) discloses deaerating a ceramic paste under vacuum, and in a separate step uses vacuum to avoid air entrapment when laminating two layers of extruded ceramic paste.
U.S. Pat. No. 4,556,526 (Yajima et al.) involves sintering a semi-inorganic material under inert conditions, which may include vacuum.
U.S. Pat. No. 4,632,686 (Brown et al.) deals with molding and melting a quartz glass article in which vacuum is used to remove air from powdered quartz as it begins to melt so as to eliminate bubbles from the glass product.